1. Field of the Invention
The present invention relates to a method of manufacturing a blazed diffractive grating and a method of manufacturing a mold for manufacturing a blazed diffractive grating.
2. Description of the Related Art
As shown in FIG. 1, a blazed diffractive grating has asymmetrical triangular grooves, and includes a blazed surface 2′ on which light 7 is incident, and a counter surface 3 adjacent to the blazed surface 2′. The blazed surface 2′ and counter surface 3 respectively tilt at predetermined angles with respect to a grating surface 4. In such a blazed diffractive grating, the diffraction efficiency must be concentrated on a given order. To achieve a high diffraction efficiency, it is required that the blazed surface 2′ is flat and its unevenness or roughness is small. A cutting volume largely influences the flatness of the blazed surface 2′. If a cutting volume in one cutting process is large, burrs and surface roughness occur on the blazed surface 2′, as shown in FIG. 2.
To decrease stress that acts on an object in forming one groove in it, therefore, a groove processing method of performing a shallow cutting process many times is adopted. Japanese Patent Laid-Open No. 2002-233912 has disclosed a groove processing method of forming an isosceles triangular groove in an object by performing a cutting process six times to suppress occurrence of burrs at the edges of the groove. In this method, a cutting process is performed twice at a position shifted to one side from the forming position of a groove, and is performed twice at a position shifted to the other side, and finally a finishing process is performed at the forming position of the groove.
To manufacture one blazed diffractive grating, for example, it may sometimes be necessary to form tens of thousands of grooves. In the conventional technique of repeatedly performing a shallow cutting process many times to form one groove, the time required to process grooves is too long, thereby reducing the manufacturing efficiency.